Rock classification for fieldwork Flashcards by Andrew Jeavons

What 3 broad categories of rock are there?

igneous
sedimentary
metamorphic

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What di crystalline rocks grow together to form?

aggregates of crystals

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What type of rock are crystalline rocks generally?

igneous or metamorphic

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Will crystalline rocks typically be more compacted or fragmented?

compacted

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What are fragmental rocks like?

minerals and rock particles that have been transported and deposited together by water, wind or ice

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What the the general characteristics of fragmental rocks?

Often rounded
Friable
Mostly sedimentary

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What does it mean when saying a rock if friable?

tends to break up

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How are igneous rocks formed?

cooling of magma

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How will the crystals typically be orientated in igneous deposits?

randomly unless from lava flow then will have flow orientatation

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What are sedimentary rocks formed from?

rock formed from sediment (solid material) moved to current position by water (less so but ice and wind)

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What might the solid material that makes up sedimentary rocks be?

other rock fragments,
mineral fragments
organic material
Precipitate minerals

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What are the general characteristics of sedimentary rocks?

Friable
fossil
rounded fragment pre-existing rock

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What is sedimentary bedding?

planar structure reflecting the sediment being ‘laid down’ in ‘beds’

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What conditions are regional metamorphic rocks formed under?

High pressure ad high temperature

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How will platy minerals grow with their orientation? (regional metamorphism)

Mica and clay etc. have preferred direction orientation right angle to principle pressure

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What does it mean saying regional metamorphism creates foliation banding?

when pressure produces alignment and may produce strong cleavage

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What will other metamorphic rocks be like?

Not regional metamorphic rocks

Either don’t contain platey minerals hence no foliation
Metamorphosed just by heat with minimal pressure (no foliation)

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What 2 broad catagories can igneous rocks be classified by?

Extrusive
Intrusive

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What are extrusive minerals like?

Fast cooling - limited crystal growth
Found cose to surface

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What will mineral formation be like for extrusive igneous rocks?

Medium to slow cooling
Medium to large crystal growth

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How can a more accurate nme be given to igneous rocks in the field?

Crystal size
Colour (proportion light to dark)

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What will the different coarse (>5mm) crystal igenous rocks be at dark, intermediate and light?

Dark- gabbro
Intermediate (50/50)- diorite or granodiorite
Light- granodiorite or granite

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What will the different medium (1-5mm) crystal igenous rocks be at dark, intermediate and light?

Dark- dolerite
Intermediate- microdiorite
Light- microgranite

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What will the different fine (1mm) crystal igenous rocks be at dark, intermediate and light?

Dark- basalt
Intermediate- andesite
Light- rhyolite

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What parameters can be used to define igneous rocks to a greater detail?

Mode of formation-Intrusive/extrusive Percentage composition Grain/crystal size Texture Colour Density

What other name can be given to other metamorphic rocks?

contact metamorphism (as will tend to form around igneous intrusion)

What will the lithology and pressure/temp be for a rock with this property: Fine grained, but hard, very fissile along strong cleavage?

Slate Low temp & pressure

What will the lithology and pressure/temp be for a rock with this property: Fine-medium grained, hard, surface may have a slight sheen due to growth of micaceous minerals?

Phyllite Greater pressure and temp than slate

What will the lithology and pressure/temp be for a rock with this property: Coarser grained with obvious mica crystals, still shows good cleavage?

Schist Greater temp and pressure than phyllite

What will the lithology and pressure/temp be for a rock with this property: Coarse grained with no cleavage, but distinct bands of light and dark minerals?

Gneiss High temp and pressure

What will the lithology of the other metamorphic rocks be with these properties: Dominated by one mineral – quartz. Crystalline, hard rock. Pale grey to buff coloured?

Quartzite

What will the lithology of the other metamorphic rocks be with these properties: Dominated by one mineral – calcite. Crystalline but softer than quartzite?

Marble

What will the lithology of the other metamorphic rocks be with these properties: Fine grained rock, may see original sedimentary bedding. Rock fractures irregularly across the bedding planes, not along it?

Hornfels

What are the three main types of sedimentary rock?

Siliclastic rock Carbonates other non-clastic rocks

what are siliclastic rocks made up of?

fragments of pre-exixting rock material

What are carbinate rocks made up of?

clastic or precipitated from solution

What will the lithology of a very coarse (>4mm) sedimentry rock by? (classfication)

conglomerate (rounded fragments) breccia (angular fragments)

What will the lithology of a coarse (2-4mm) sedimentry rock by? (classfication)

granulstone (gritstone)

What will the lithology of a medium (0.006-2mm) sedimentry rock by? (classfication)

arenites (sandstones)

What will the lithology of a fine (<0.006mm) sedimentry rock by? (classfication)

mudrocks

What fractions can mudrocks be splt into?

Medium fine (siltstone) Fine (mudstone) Very-fine (claystone)

How can medium-grain siliclastic rock arenite be futher split?

based on the nature of its fragments

What are 2 examples of how fragments can split arenite? | There are 4 to choose from

Fragment quartz grain- quartz arenite >25% feldspar- arkose rock fragment not mineral- lithic arenite significant muddy matrix- graywacke

What is a % abundance chart used for with classification?

visually estimating poportion of particular component

What is the easiest way to identify different carbonates?

all tend to react with hydrochoric acid to varying degrees

What will a carbonate be like if dominantly calcium carbonate (CaCO3)?

limestone

What will a carbonate be if dominantly calcium or magnesium carbonate (Ca,Mg)CO3?

dolomite

What is a rock matrix?

the relative finer grained sized sedimentary particles in between coarse-grained particles

What is the matrix of most carbonate rocks made up of?

limestone mud particles 4um in size (micrite)

What is cement? (carbonates)

lithied post deposition binding material and is precipitated from moving solutions in the formation

What are the crystals of most cement carbonate rocks made up of?

Spar (sparite)- CaCO3 crystal

What are allochems?

grains or clasts within carbonate rocks

What are the most imporant type of allochems?

Bioclasts Clasts Ooids Oncoids Peloids

What are bioclasts like?

fragmentary or whole organisms

What are clasts like?

interclasts (intraformational) Lithoclasts (extra clasts?

What are Ooids?

coated grains with a regular calcareous cortex and a nucleus of varying composition

What are oncoids like?

coated grains with an irregular calcareous cortex

What are peloids like?

sand-sized grains of microcrystalline carbonate Internally structureless (feacal origin)

# Both created in 1962 What 2 carbonate classification graphs are there?

Dunham Folk (packing and sorting)

What can can be done with the matrix and allochem to produce a rock name?

Combined allochem prefixes are listed in descending order of abundance

How can bedding/ lamination be classified?

Spacing up to 10mm= lamination 10 to >1000mm= bedding

How do pyroclastic form?

disintegration of magma, as gases are released by decompression and then ejected from a volcanic vent either in air or beneath water

What is the most extreme form of pyroclastic?

Nuée ardente

What is a Nuée ardente?

turbulent, fast-moving cloud of hot gas and ash erupted from a volcano.

What are epiclasts?

Sedimentary rocks from lithic clasts and minerals released by ordinary weathering processes from pre-existing consolidated rocks

What are volcanic epiclasts?

volcanic composition from erosion of volcanoes or ancient volcanic terrains

What is a lahar?

volcanic mudflow

What is Tuff?

consolidated volcanic ash